During molding operations, there is a need to vent air from cavities within a mold tool that define the object to be molded. As a molding compound enters the cavity, it compresses any air in the cavity. If there is no means for the air to escape, the resulting molded object will have defects caused by the trapped air pockets.
A known method for venting air from mold cavities is to provide the mold tool with venting holes. One application of the use of venting holes is in the semiconductor industry, where semiconductor devices are often transfer molded to create plastic encapsulating bodies which protect sensitive semiconductor elements, such as integrated circuit die, from environmental influences. In molding a typical plastic package about a semiconductor element, air is often vented out of the package cavity of the mold through very narrow recesses or troughs formed in the mold tool, commonly located at the comers of the package cavity. These recesses are made shallow enough to prevent the molding compound from seeping through the recesses. The recesses lead to more voluminous cavities in the mold or extend to the periphery of the mold where air is released into the atmosphere.
Molding one particular type of semiconductor package, known as a molded carrier ring (MCR), poses a venting problem which cannot be handled by traditional corner venting recesses. A typical semiconductor device 10 packaged in an MCR is illustrated in FIG. 1. Device 10 includes a molded ring 12 and a molded package body 14. A semiconductor element (not illustrated in FIG. 1), such as an integrated circuit die, is encapsulated by package body 14. A plurality of conductive leads 16 of a conventional metal lead frame (not fully illustrated) extend from the package body to the carrier ring. The leads are electrically coupled to the internal semiconductor element by conventional means, for example wire bonds (not shown). In prior processes, ring 12 and package body 14 are formed using a transfer molding technique wherein a resin molding compound is introduced into a ring cavity by way of a first mold gate 20. The resin then flows around the ring, as the arrows in FIG. 1 indicate, and is introduced into a package cavity by way of a second mold gate 22. Because the ring cavity is filled before the package cavity and because the ring cavity completely surrounds the package cavity, air which is forced out of the package cavity must be vented through an area of the mold tool contained within the ring.
In existing MCR transfer molding systems, a venting hole is formed in a bottom platen of a mold tool, in between the package cavity and the ring cavity. Air is transferred from the package cavity through venting recesses in the corners of the package cavity, as in traditional plastic packages without a carrier ring. But rather than being vented to the atmosphere or another large cavity within the mold, the air is directed to the venting hole, having a diameter on the order of 2.0 mm. The venting hole is sufficient to vent the air from the package cavity during molding of an MCR package; however, this design poses other manufacturing problems. One problem occurs in automated molding equipment which automatically transfers lead frames through the mold tool. If such equipment happens to misalign a lead frame within the mold tool, the mold platens may not dose properly. Upon shooting the mold compound, which the equipment will do if it cannot detect the misalignment, the mold compound will not only fill the package and ring cavities, but will also fill the venting recesses and venting holes because the package and ring cavites of the mold platens are not properly dosed. If this occurs, the mold tool must be shut down for cleaning, causing lengthy and costly down time.
Another problem with conventional mold designs for MCR packages is that these molds cannot be compression cleaned. Periodically, mold tools must be cleaned to remove residue which builds up on the mold platens. In semiconductor manufacturing, two mold cleaning methods are prevalent, transfer cleaning and compression cleaning. In a transfer cleaning process, a pellet of cleaning resin, for instance melamine, is shot through the mold in the same manner as a pellet of molding compound is shot to form the package. Thus, a dummy lead frame is positioned in the mold tool and a package and a ring of the melamine compound is formed about the lead frame as if it were an actual semiconductor device lead frame (only the melamine package and ring are typically white whereas semiconductor grade molding compound is black). In a compression cleaning process, the melamine, or other cleaning resin, is simply spread over the bottom platen of a mold tool. The top and bottom mold tools are brought together to compress the melamine. Since there is not a lead frame in the mold tool, the melamine spreads across the entire top and bottom platen, filling every cavity. If this compression cleaning technique were used in a conventional MCR mold tool, the melamine would also fill the venting hole, thereby requiring extended down-time. Therefore, existing MCR tools are limited to the transfer cleaning method. However, transfer cleaning has disadvantages as compared to compression cleaning, in that transfer cleaning 1) must be done more frequently; 2) is less effective at removing residue; and 3) is more costly.
Accordingly, there is a need for a method for molding semiconductor devices, especially for molding MCR devices, that enables the use of compression cleaning techniques and that reduces the need for mold down-time if lead frame misalignment occurs.